Music is a language. Simple as that. We learned how to talk long before we learned to read and write. Learning to speak is fairly easy, we simply copy what we have heard, and learn how to use it for ourselves. Many "musicians" have this level of musical ability. They have learned much of the "dialogue" and can use what they have heard to carry on a musical "conversation" with other musicians. There are many pros who rely on this basic level of ability and it serves them well. They can "write" songs by simply writing the lyrics and the basic chords on top and remember the arrangements. This is like learning to speak and learning words and remembering the language.
However, If you truly wish to better understand "how" the language is created than you learn to read and write and the syntax and rules involved with language. This happens to us by going to school. We learn the rules of language and how to use them. This knowledge is not absolutely necessary to be able to "communicate" but it definitely gives us the ability to communicate much more effectively.
The same is true with music harmony and theory. By learning these basic concepts of musicianship we become better at conveying our musical ideas and exposed to a much wider range of "dialogues" and "vocabularies".
Make no mistake, all musicians do learn concepts of harmony and theory simply by using their ears and listening to music over and over. This "trains" our ears to recognize certain "phrases" and "vocabulary" which we copy and learn to use. Just like a child learns to speak. They might not have taken any formal lessons in music but they have learned the concepts of musical dialogue on a limited level.
You can go through life with a basic high school education and understanding of language just fine. But there is more to learn, a LOT more! Similarly you can function on a basic musical level with just a comprehension of major, minor, simple chord progressions, basic scales usage, etc. But if you truly wish to take your music, and ability, to a higher level you will want to learn more about the language of music, how it is constructed, how the various elements of it are used.
You can do this in a variety of ways such as taking lessons with a good qualified local instructor, taking classes in high school and college, etc. You do not need to pursue a degree in music from a university to gain a lot of useful knowledge. Simply opening your mind with lessons and some classes is a great way to vastly improve your knowledge. You have been learning to use the basic ideas of music and you will continue to gain a better understanding of it simply by using your ears and listening to what is going on. However, at some point you may wish to take it to a higher level and gain a deeper understanding of the how and why things are the way they are. Good luck my friend.